The present invention relates to a method for regulating current and, more particularly, to a circuit and method for limiting the current flow in a semiconductor device.
There are many applications in which it is desirable to protect a transistor by limiting the maximum current flow in the collector-emitter path of the transistor. For instance, when a transistor is operating in its active region, a large voltage drop across the collector-emitter electrodes in the device could cause the transistor to be damaged or even destroyed because the power dissipation characteristics of the transistor may be exceeded. One way to prevent this occurrence is to limit the current through the transistor to a maximum value which is well within the power handling capabilities of the device.
One prior art technique for protecting high gain transistors from conducting high currents simultaneously with large voltage potentials applied across the collector-emitter junctions thereof is to utilize a sense resistor between the emitter and ground reference in conjunction with a feedback circuit coupled between the emitter and base of the transistor. In a typical application, the feedback circuit may be an additional transistor having its base and emitter coupled across the sense resistor and its collector connected to the base of the transistor to be protected. Hence, whenever the current through the high gain transistor produces a voltage across the sense resistor sufficient to turn on the feedback transistor, base current to the high gain transistor is stolen through the collector-emitter of the feedback transistor thereby limiting the current in the output transistor.
However, the above described prior art circuit suffers from two disadvantages that may not be acceptable in some applications requiring protection of output transistors commonly found in operational amplifiers, comparators, automotive circuits, etc. First, the voltage drop across the sense resistor is at least equal to the base-to-emitter voltage, V.sub.BE, of the feedback transistor which reduces the V.sub.SAT characteristics of the circuit comprising the protected transistor. Thus, the maximum output voltage swing of this prior art circuit is limited which may not be tolerable. Secondly, the addition of the sense resistor causes a loss in the g.sub.m of the transistor amplifier which is not desirable in many cases.
Another prior art method for limiting current in an output transistor is illustrated in FIG. 1 herein. This prior art circuit comprises output transistors 10 and diode 12 and is commonly referred to as a current mirror. The anode and cathode of diode 12 is connected to the base and emitter respectively of transistor 10. Transistor 10 is illustrated as having an emitter area N times the area of the anode region of diode 12 as understood. A preceding or intermediate amplifier stage comprising transistor 14, which is adapted to receive an input signal at base 16, is coupled at its collector to the base and anode of transistor 10 and diode 12 respectively. A current source 18, supplying a current I, is connected between power supply V.sub.CC and the collector of transistor 14.
In operation, the output current flowing in transistor 10 is at all times equal to NI', where I' is the current flow in diode 12. Thus, the maximum current flow at the collector of transistor 10 is limited to NI. Hence, it is seen that this circuit suffers from a severe loss of beta, the forward current gain factor of the transistor.
Therefore, a need exists for a circuit and method for limiting the current in a semiconductor device which circuit exhibits good VSAT characteristics and which does not suffer severe loss in the gain thereof.